Interpreting blobpage average fullness

You can analyze the output of the oncheck -pB command to calculate average fullness.

The first simple large object listed in Determine blobpage fullness with oncheck -pB output is stored in the blobspace blobPIC and requires one 6144-byte blobpage. The blobpage is 51 to 75 percent full, meaning that the size is between 0.51 * 6144 = 3133 bytes and 0.75 * 6144 = 4608. The maximum size of this simple large object must be less than or equal to 75 percent of 6144 bytes, or 4608 bytes.

The second object listed under blobspace blobPIC requires two 6144-byte blobpages for storage, or a total of 12,288 bytes. The average fullness of all allocated blobpages is 51 to 75 percent. Therefore, the minimum size of the object must be greater than 50 percent of 12,288 bytes, or 6144 bytes. The maximum size of the simple large object must be less than or equal to 75 percent of 12,288 bytes, or 9216 bytes. The average fullness does not mean that each page is 51 to 75 percent full. A calculation would yield 51 to 75 percent average fullness for two blobpages where the first blobpage is 100 percent full and the second blobpage is 2 to 50 percent full.

Now consider the two simple large objects in blobspace bspc1. These two objects appear to be nearly the same size. Both objects require two 2048-byte blobpages, and the average fullness for each is 76 to 100 percent. The minimum size for these simple large objects must be greater than 75 percent of the allocated blobpages, or 3072 bytes. The maximum size for each object is slightly less than 4096 bytes (allowing for overhead).

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